Method idSet::subId is not forwarded for arbitrary codimension

Currently, there is an asymmetry in the inferface for index sets and id sets: IndexSet::subIndex may be implemented for entities of arbitrary codimension, while the IdSet interface only forwards subId codim-0 entities.

Unfortunately, simply adding the corresponding subId method the usual way will break grid implementations (they would have to implement subId< cc > instead of subId). Therefore, this issue is related to #28.